How a hedgerow unit is counted for biodiversity net gain
The hedgerow module of the statutory metric works in kilometres, needs its own 10 per cent gain, and treats a line of three trees differently from a single tree in a lawn.
Hedgerow units sit in their own module of the statutory biodiversity metric, measured in kilometres rather than hectares, and the 10 per cent gain required under the Environment Act 2021 has to be achieved in that module on its own. Remove 60 metres of species-rich native hedgerow for an access splay and no amount of wildflower meadow elsewhere on the site will fix it. Individual trees are handled differently again, and whether a tree counts in the hedgerow module or the area module depends on whether it stands in a line.
That distinction decides more small-site layouts than most applicants realise.
How the metric treats hedgerows separately
The hedgerow module takes length in kilometres and multiplies it by distinctiveness, condition and strategic significance. After development, the same calculation adds multipliers for difficulty of creation or enhancement and for time to target condition, plus a spatial risk multiplier where delivery is off site.
Distinctiveness is set by hedgerow type, and the metric's types are finer-grained than a typical site survey. Species-rich native hedgerow, native hedgerow with trees, native hedgerow, ecologically valuable line of trees, line of trees, and species-poor variants. A survey note reading "mixed native hedge, gappy in places" does not give the metric enough to work with, and the ecologist will have to go back.
Two definitions get confused. The metric's hedgerow categories are not the same as an important hedgerow under the Hedgerows Regulations 1997, which uses its own criteria of age, length and species or historical association, and which requires a removal notice to the local planning authority before a hedgerow in the countryside is grubbed out. A hedgerow can be low distinctiveness in the metric and still be important under the 1997 Regulations. Both tests have to be run.
Where individual trees appear in the calculation
A tree standing alone is not a hedgerow. The metric records individual trees by canopy area as a habitat parcel in the area module, with the canopy area derived from stem diameter using the conversion in the current user guide. Three or more trees in a line, with canopies that read as a linear feature, fall into the hedgerow module as a line of trees or an ecologically valuable line of trees.
That boundary has a practical consequence. Felling two mature oaks in a field hedge reduces the hedgerow condition score and may change the hedgerow type from native hedgerow with trees to native hedgerow. Felling two mature oaks in a lawn produces a loss of area units instead. The arithmetic, the compensation and the trading rules that apply are all different.
Check the version of the metric you are using, because the treatment of trees has been refined between releases and an older version will produce a different answer from the one the authority runs.
Veteran trees and ancient woodland sit outside the metric entirely as irreplaceable habitat. No number of units compensates for them, and the compensation has to be agreed with the authority on a bespoke basis.
Condition assessment for a hedgerow
Condition is where a hedgerow result is won or lost, and it is assessed against a published criteria sheet rather than by professional impression.
The criteria cover height and width, gappiness along the length, gappiness at the base, the presence of an undisturbed margin of perennial vegetation, invasive species, and signs of damage from stock, machinery or herbicide. A set number of criteria must be met for moderate condition and a higher number for good condition, and the sheet records which criteria failed.
Three criteria are routinely failed and two of those are fixable cheaply.
Gaps are the most common failure, and gapping up with whips of matching native species is the cheapest condition improvement available on a small site. Base gappiness, usually caused by shading or by stock, responds to laying or coppicing on a rotation. A missing perennial margin is a management change rather than a capital cost: stop mowing or spraying a strip and the criterion is met within a season or two.
Height and width failures are harder, because they need years of growth and a management regime the authority can hold you to for 30 years.
A photographic record of each hedgerow section, with the completed condition sheet, belongs in the submission. An assertion that a hedge is in moderate condition, with no sheet behind it, gets queried.
Retaining a hedgerow and the uplift it earns
Retention plus enhancement almost always beats removal plus creation in the metric, and the reason is the time to target condition multiplier.
A new hedgerow planted as whips takes years to reach good condition, and the metric discounts the units accordingly. An existing hedgerow lifted from poor to moderate condition by gapping up and establishing a margin reaches its target in a far shorter period, so the discount is smaller. The same intervention also avoids the difficulty multiplier applied to creation.
So the design sequence is: keep the hedge, take the access through the widest existing gap, improve what remains, and only then look at new planting to make up the balance.
Setting the layout back from the hedgerow is what makes the enhancement deliverable. A hedge with a garden fence 300 millimetres from its base cannot have an undisturbed perennial margin, cannot be laid without entering private gardens, and will not be maintained to the standard the management plan promises. A 3 metre margin in public or managed open space is the difference between a hedgerow that holds its condition for 30 years and one that does not.
The Landscape Strategy is the document that carries that margin through to the planting and maintenance specification.
Losses that are hard to replace on site
Some hedgerow losses cannot be made good within the red line, and recognising them early saves a wasted application.
A species-rich native hedgerow with mature trees is high distinctiveness, and the trading rules require like-for-like or better compensation rather than units of any type. Creating a species-rich hedgerow takes decades and the metric knows it.
A hedgerow on a boundary in third-party ownership can be measured as a loss while being impossible to enhance, because you have no right to work on it. Where that applies, say so in the metric notes and in the gain plan, and look for off-site units in the same module.
Off-site hedgerow units are thinner on the ground than off-site area units in most catchments, and the spatial risk multiplier discounts delivery outside the authority area or the relevant national character area. Statutory biodiversity credits remain the last resort, priced to be unattractive.
Interaction with tree protection conditions
Two sets of professionals produce two sets of drawings for the same site, and they conflict more often than they agree.
The arboricultural side produces a BS 5837:2012 survey, a tree constraints plan, root protection areas and a tree protection plan, usually carried by the tree survey and arboricultural impact assessment. The ecology side produces habitat parcels, hedgerow lengths and a planting plan to deliver the gain.
Four conflicts recur. New hedgerow or tree planting shown inside a root protection area where the method statement forbids excavation. Protective fencing lines that cut through the margin the ecology plan relies on. Planting specifications that disagree on species and stock size. And replacement tree planting required by condition being double-counted as biodiversity gain, which it cannot be where the planting is compensation for a loss already recorded in the metric.
Reconcile the two plans before submission and say in the covering letter that you have. Tree protection fencing has its own specification and timing requirements, set out in tree protection fencing: specification and timing, and replacement planting conditions bind for longer than most applicants expect, as tree replacement conditions and how long they bind you explains.
Evidence the authority checks
A hedgerow result gets checked against six things, and the check takes an ecologist about twenty minutes.
- The metric version number, stated and current.
- The habitat plan with each hedgerow section numbered and its length dimensioned, matched to the metric rows.
- The completed condition sheets, with the failed criteria visible rather than only the resulting band.
- Photographs of each section, dated.
- The surveyor's name and competency, and the survey date against the season.
- The 30-year management plan, naming who maintains the hedgerow and how the margin is protected.
The metric and the biodiversity gain plan must be completed by a competent person under the Environment Act 2021 regime, and the plan has to be approved by the local planning authority before development begins. A drafted Biodiversity Net Gain Assessment is a brief for that person and a structure for the submission, rather than a replacement for the survey or the condition assessment.
Enter your postcode on PlanScope to see which tree and ecology reports a site is likely to need.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
Are the metric's hedgerow types the same as an important hedgerow?
No. The statutory metric classifies hedgerows by type and condition for the purpose of calculating units. An important hedgerow under the Hedgerows Regulations 1997 is a separate test based on age, length and species or historical association, and removal requires a notice to the local planning authority. A hedgerow can score low in the metric and still be important under the 1997 Regulations.
Where does a single tree appear in the metric?
A tree standing on its own is recorded in the area module as a habitat parcel, with its area derived from canopy extent using the conversion in the current metric user guide. Three or more trees in a line are recorded in the hedgerow module as a line of trees. The treatment has been refined between metric versions, so check the version you are using.
What is the cheapest way to improve hedgerow condition?
Gapping up with whips of matching native species, and establishing an undisturbed margin of perennial vegetation by stopping mowing and spraying along a strip. Both address criteria that are commonly failed, and the margin costs a management change rather than capital. Height and width failures take years of growth and a maintenance regime the authority will hold you to.
Why does retaining a hedgerow score better than replanting one?
Because the post-development calculation applies multipliers for difficulty of creation and for time to target condition. A new hedgerow planted as whips is discounted heavily while it establishes, whereas lifting an existing hedgerow from poor to moderate condition reaches its target sooner and avoids the creation multiplier entirely.
Can replacement tree planting required by condition count as biodiversity gain?
Not where the planting is compensation for a loss already recorded in the metric, because that would double-count the same intervention. Reconcile the arboricultural replacement schedule with the ecology planting plan before submission, and state in the covering letter which planting delivers gain and which discharges the tree condition.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.